Siliconizing processes



March 23, 1950 A. s. HENDERSON ET AL 2,501,051

SILICONIZING PROCESSES Filed Feb. 11, 1943 2 SheetsSheet l ATTORNEY March 1950 A. s. HENDERSON ET AL 2,501,051

SILICONIZING PROCESSES Filed Feb. 11, 1943 2 Sheets-Sheet 2 Patented Mar. 21, 1950 UNITED STATES rA'rsN'r OFFICE SILICONIZING PROCESSES Ashland S. Henderson, Edward E. Slowter, and Bruce W. Gonser, Columbus, Ohio, assignors, by mesne assignments, to The Duriron Company,- Inc., Dayton, Ohio, a corporation of New York Application February 11, 1943, Serial No. 475,560

. lClain1s. l U

This invention relates to. siliconizing processes and products thereof; and it relates specifically to the production on metal articles, especially ferrous metal articles, of an adherent coating or case containing silicon which, besides being resistant to abrasion or mechanical wear, is particularly efiective in protecting the underlying body or core of the article from corrosion. Said coating or case is dense and highly resistant to corrosion, withstands thermal and mechanical shocks well, and is firmly united to said body or core by a bond that is, substantially unaffected by such shocks even when of considerable magnitude. .As here used,v the expression thermal shaped ferrous metal articles with certain ex-.

posed or working surface portions thereof having a composition very much higher in silicon than the underlying body or core portions of the articles and firmly bonded thereto. It is common knowledge that silicon-iron alloys, such as Duriron which contains in the neighborhood of 14 per cent silicon, are not only resistant to abrasion or mechanical wear but are also highly resistant to certain acids and otherv corroding agents. But such alloys are characteristically brittle, of relatively low strength and toughness, and difficult to machine. Therefore, although the exceptionally high corrosion resistance of these alloys has resulted in their extensive industrial use for certain purposes, their field of usefulness would be much wider if articles composed wholly of them were not so poorly resistant, relatively, to shock or stress of substantial magnitude. In some cases, also, the cost of articles made entirely of a high-silicon alloy of iron or steel has been prohibitively high for the use intended.

Accordingly, attempts have been made heretofore to produce an article having a surface coating or case of a high silicon-iron alloy adhering to a strong tough core of iron or steel, by first fabricating the required article or part substantially in desired final form from iron or steel of the desired strength, toughness and other mechemical properties, and then endeavoring to siliconize the surface of the article or some selected portion of such surface in a manner intended to cause penetration or difiusion of a siliconizing agent into the metal to a suflicient depth to proand variable as to'render them of virtually negliv gible importance commercially."

While the articles 1 produced in such prior attempts possessed in some instances a siliconized surface offering somewhat enhanced resistance to abrasion or mechanical wear, the silicon-iron coating or case afforded insufilcient protection of the underlying core "against acidcorrosion. Aside from other defects, thesilicon content of the case produced was generally too low to render the case material-"itself satisfactorily resistant to corrosion. In this connection, it may be stated that, as a general rule, itis highly desirable that the case of a siliconizedarticle consist at least largely of a silicon-iron alloy, or a plurality of such alloys, containing in" excess of 14 per cent silicon; because 13-- to 14 per cent silicon alloys may. show corrosion losses several hundred times as great as thoseshown by 14 and 14.5 per cent alloys under similar conditions; For best results as regardsresistanceto attackbyoxidizing acids such as sulphuric and'nitric; the silicon content of such alloy or alloys in the case material, to a substantial depth beIoW the Out'er surface of the case, should therefore be in excess 0f14.4 per cent; and should most desirably be above about made of-low and mediumcarbon steels, and with articles of high carbon steels as well (the carbon of high carbon steels especially tends strongly to migrate inwardly during the siliconizing treatment), the average or overall silicon content of the siliconized case should most" desirably be at least 14 per cent or higher, With other ferrous articles, such as those composed of malleable and cast irons, however, the much'higher carbon content of which (often'fi 'or '4 per cent, e. g), as applicants have di's'covered',does not migrate inwardly to a substantial extent when such articles undergo siliconizing treatment, it means that the siliconized case may have an average or overall silicon content of less than 14 per cent and yet have excellent corrosion resistance, since the carbon and other minor "constituents or fimpurities present in the case apparently actmerely to dilute the high-silicon alloy or alloys of iron without materially lessening the efiect of such alloys in rendering the articles resistant to corrosion.

Siliconized cases heretofore obtained were also quite commonly so porous as to be readily penetrable by liquid or gaseous eorrodin agents and hence of little or no practical value in rendering the cased article corrosion resistant; said cases, whether porous or not, being commonly rather easily cracked and thus rendered useless for protection against corrosion. Moreover, due to weak bonding of case to core, the prior art ewes were generally characterized by poor adherence and pronounced tendency to spall off, this being evident upon the articles being subjected to substantial thermal or mechanical shock such as they might normally be expected to encounter under actual service conditions.

So far as we are aware, only one siliconizing procedure has found any practical application whatever in industry heretofore, and its usefulness is extremely limited. That procedure has involved embedding the article in a mass of comminuted solid material, such as silicon, ferrosilicon or silicon carbide, heating the embedded article to a relatively high temperature and introducing into said mass a suitable reagent such as chlorine gas. The chlorine reacts with the silicon carbide (e. g.) and silicon is said thereby to be diffused into the article undergoing treatment. While deposits of silicon on articles of ferrous metal in particular have been obtained in this manner and have conferred upon said articles some increased resistance to mechanical wear, recognized standard tests show that the resistance of such articles to corrosion has been far from satisfactory. This low resistance to corrosion may be due in part to a certain degree of porosity which characterizes a siliconized case produced in the manner described and which, it is asserted, renders the case capable of absorbing and retaining substantial amounts of oil. It may also be due in part to surface irregularities in the case resulting from spot contacts between the aforesaid comminuted solid material in which the article was embedded and the surface of the article. Also, the bond or union between the case and the unsiliconized metal base or core is not as strong and resistant to shock or stress as is desirable.

Said prior procedure is especially inapplicable, moreover, to siliconizing articles composed of gray cast iron because of the marked tendency for excessive swelling to occur during the treatment. This swelling, which is to be distinguished from mere increase in overall dimensions of the original article due to silicon deposition, weakens the bond between case and core, thus leading to excessive spalling of the case under service conditions, with resultant loss of corrosion resistance and other disadvantages. Because of their low cost and ready machinability, ordinary gray irons are often desirable materials for the manufacture of many articles which it would also be highly desirable to render corrosion-resistant by siliconizing.

The prior procedure aforesaid is also not well adapted for siliconizing alloy irons and steels, especially those containing substantial percentages of chromium; nor for siliconizing various non-ferrous materials, such as nickel and copper.

A general object of the present invention is to avoid the difficulties and objections characterizing prior proposed siliconizing procedures, and to produce siliconized articles superior to those heretofore obtainable, by means of a novel process which does not require embedding the 4 articles in comminuted solids and which ensures the production of a dense, high silicon-iron case or coating which is strongly adherent to the metal of the base or core, rendering the article not only resistant to abrasion or mechanical wear, but also highly resistant to corrosion as determined by tests customarily employed in the art; the coating being so firmly adherent as to withstand substantial thermal shock successfully.

A further important object of the present invention is to provide such a process which is relatively easy to carry out and which can be positively controlled to give dependably satisfactory silicon-containing protective surface layers or coatings strongly resistant not only to abrasion but also to corrosion and to thermal and mechanical shock.

A more specific object of the invention, and one of primary importance, is to enable the production, in a practical way, of a metal article, more especially a ferrous metal article, having a firmly adherent case or coating of a siliconmetal alloy, specifically a silicon-iron alloy, which is higher in silicon than cases or coatings heretofore produced on metal articles, that is, higher than the maximum of about 14 to 15 per cent silicon heretofore rarely characterizing such coatings; but it is to be understood that the invention is in no sense limited to providing metal articles with cases higher in silicon than such previous maximum.

Further specific objects of the invention are: To product on a base or core of cast iron, particularly gray cast iron, a high-silicon case or coating which is strongly adherent to said base or core; and to produce such cases or coatings on bases consisting of alloy irons and alloy steels generally, including those containing chromium, as well as on non-ferrous metals, particularly copper.

Other and further objects of the invention will appear from the disclosure hereinafter.

While the present invention is more especially concerned with the production of articles having an iron or steel base or core with a case or coating of a relatively high-silicon alloy of iron bonded thereto, possessing the characteristics hereinabove emphasized, and while the further disclosure and explanation hereinafter given will be directed more particularly to such articles, it is to be understood that, in its broader aspects, the process is applicable for coating articles composed of other metals, such as nickel and copper; and, under some conditions, for coating even non-metallic articles of a sufficiently refractory nature.

The literature of the art extending over a long period of years contains various proposals to siliconize metal articles by treating them with siliconizing reagents wholly in gaseous form, without having comminuted solids in contact with the article; but the results obtained were so erratic that none of such gas treatments heretofore proposed has been put into practical use. For example, it has been proposed to deposit silicon on iron by the use of a silicon chloride, specifically silicon tetrachloride (SiCh), employed in vapor or gaseous form and, optionally, in the presence of another gas, such as nitrogen or hydrogen which is primarily intended to serve merely either as an inert carrier or diluent for the silicon chloride or, in the case of hydrogen, because of its reducing character, to guard against oxidation of the iron by any oxidant accidentally present in the treating atmosphere. It has been pointed out that such use'of hydrogen renders-theaction of the-silicon chloride on the iron more intense but less regular; it being. also suggested that this may possibly be due to some reduction of the silicon, chloride by the hydrogen, in additionto the direct reaction between the ironand the silicon chloride. As typical of such prior art literature, reference is here made to AustrianPatent No. 75,151, and corresponding German Patent No. 302,305, both granted in 1917 to Bosnische Elektricitats A. G.; and to a paper by .Sanfourche .appearing in Comptes Rendus, vol. 183 (1926), pages 791793.

The process of the present invention comprises maintaining the article to besiliconized at a sufiiciently high reaction temperature in a flowing treating atmosphere containing, essentially, gaseous silicon tetrachloride (or other chloride of silicon e. g. trichlorosilicane, SiI-ICls) and hydrogen. The treating atmosphere may consist substantially only of gaseous silicon tetrachloride and hydrogen, as free from other gases and vapors as is feasible in practice. Or it may contain a further gas or gases serving merely as a diluent or carrier of the two essential components. Such further gas or gases may be present either because added as such or because incidental to the employment of the hydrogen in a form that is available commercially at relatively low cost. Nitrogen is an example of such a diluent carrier gas, and cracked ammonia (a mixture of nitrogen and hydrogen commonly in proportions approximating 1 part of nitrogen to 3 parts of hydrogen by volume) is an example of a relatively cheap commercial product satisfactory to employ in supplying the necessary hydrogen component of the treating atmosphere.

The present invention is based upon the discovery that in order to produce on a metal article a silicon-containing coating Or case which will be satisfactorily resistant to corrosion and to thermal shock, by a siliconizing procedure which employs a gaseous mixture comprising a silicon chloride, such as silicon tetrachloride, and hydrogen, it is essential that the ratio of these two components one to another in the treating atmosphere be maintained within a certain range of ratios, that between definite minimum and max mum limiting ratio values.

While, in the present disclosure, reference is made more particularly to the use of silicon tetrachloride as the silicon-containing gas component of the treating atmosphere, it is to be understood that other chlorides of silicon, such as silicon trichloride (SlCls or SisClc), ma also be employed. In fact, commercial silicon tetrachloride, which is satisfactory to use in practicing the invention, commonly contains up to about 5 per cent of silicon trichloride, as well as some silicon hydride or, more probably, trichlorosilicane.

From applicants observations, it appears that hydrogen must be looked upon, not merely as a reducing agent to protect against oxidizing effects of any oxygen, either in elemental or combined form, which may have entered the system accidentally, nor merely as an intensifier or acceler- I ator of the reaction between iron and silicon chloride. On the contrary, it is now evident that, when a ferrous metal article is heated in an atmosphere of hydrogen and silicon tetrachloride to a temperature high enough and for a sufficient time to bring about a reaction between the iron and the silicon chloride, the reactions involved are more complex, or their interrelation is more complex, than has been supposed heretofore by prior workers in the art; also that inorder to produce satisfactory siliconizedarticles consistently, it is essential that the progress of the respective reactions aforesaid be constantl controlled and coof silicon and iron. But wehave found that, by

varying the ratio of hydrogen to silicon tetrachloride in the reaction mixture under properly controlled conditions, the article may be caused eitherv to gain or to lose weight in the siliconizing treatment, depending upon the value at which the ratio is maintained during the treatment. It would therefore seem reasonable to explain our discovery of the extreme importance of the role played by hydrogen-silicon chloride ratio, when treating a ferrous or other reactive metal article by the novel process herein disclosed, upon the supposition that at least two reactions are always taking place: (1) the reaction between the metal (e. g. iron) and the silicon chloride, which brings about replacement of iron by silicon, sometimes hereinafter referred to as the replacement reaction; and (2) the reaction between the hydrogen and the silicon chloride, which brings about direct deposition of silicon, sometimes hereinafter referred to as the deposition reaction. In thepractice of the present invention, the by-products of those reactions, ferrous chloride and hydrochloric acid, are continuously swept out of the reaction zone in gaseous form under the prescribed conditions of operation.

Assuming treatment of a ferrous article with silicon tetrachloride, the two reactions in question may be stated in idealized form as follows:

SiCl4+2F=Si+2F6C12 (Replacement) SiCl4+2H2=Si+4HC1 (Deposition) above assumptions are correct or not, the fact remains that any attempt to carry out a gaseous siliconizing process of the general type in question, employing silicon tetrachloride and hydrogen in a ratio, one to the other, that is outside the definite range of values hereinafter specified as the Widest permissible, although it may produce some siliconizing of the article, will result in failure to produce a durable coating or case that will satisfactorily meet the generally accepted commercial standards or requirements as regards resistance to thermal and mechanical shock and protection of the coated metal against corrosion, such protection being dependent not only upon the density and integrity of the case itself but also upon the integrity of its bond or union to the underlying metal core.

article at a rate high enough to prevent any concentration of byeproductsof the reactions suf- 75 ficient to interfere with the main reactions desired. The gas mixture entering the reaction zone must also be regulated by controlling the supply of the required constituents thereof in such manner as at all times to maintain the ratio of hydrogen to silicon tetrachloride within the herein-designated limits in such entering mixture. Provided the rate of flow is above the minimum required to maintain such required ratio in the treating atmosphere, to prevent objectionable concentration of the by-products in the reaction zone, and to ensure the presence of sufficient hydrogen and silicon tetrachloride in proper proportion operatively adjacent the article or articles to produce the desired coating thereon, the rate of flow may be relatively slow or rapid without materially affecting the character of the coating produced.v But, obviously, an unnecessarily high rate of fiow will carry off and thereby waste silicon tetrachloride which has not had time to react, and hence will be uneconomical. In particular, the rate of flow should be sufficiently high in any event to ensure sweeping out of the reaction zone the ferrous chloride byproduct of the replacement reaction, whereby to minimize the possibility of detrimental occlusion of ferrous chloride in the siliconized case produced on the article.

The process of our invention, as already indicated in part, consists essentially in subjecting the article to be coated to the action of a nonoxidizing treating atmosphere containing silicon tetrachloride (e. g.) and hydrogen in such proportions relative to each other that the weight of free hydrogen present is not less than 0.02 per cent, and not more than 4.0 per cent, of the weight of silicon tetrachloride present, the reaction zone being maintained at a temperature of at least about 1400 F. but below the fusion point of the article; and continuing the treatment in the reaction zone for a sufiicient time to give the desired thickness of siliconized case. Within the percentage range just specified, certain narrower ranges are found to give optimum results, as will hereinafter appear. The figures just given presuppose employment of commercial silicon tetrachloride conforming approximately to the specifications hereinabove set forth; and reference in the appended claims to silicon tetrachloride is to be understood as signifying such a commercial product.

When coating a ferrous metal article in accordance with the principles of the invention in what is now regarded as the best embodiment of the invention, and assuming employment of a good grade of commercial silicon tetrachloride, the article is heated to a temperature between about 1700 F. and 2000 F. for three hours or more, while maintaining the proportion of free hydrogen in the gaseous mixture entering the reaction zone at from 0.06 to 1.2 per cent, by weight, of the silicon tetrachloride, said free hydrogen constituting at least 50 to '75 per cent by volume of the gaseous medium associated with the gaseous silicon tetrachloride to form the siliconizing atmosphere, and the other necessary conditions hereinbefore explained being maintained. While it is possible to obtain acceptable results, in practicing the present process, when free hydrogen constitutes as little as 10 per cent by volume of the hydrogen-containing gaseous medium associated with the silicon chloride in the entering mixture aforesaid, nevertheless higher percentages of free hydrogen in said medium, such as those above specified, are better and are usually to be recommended in practice as contributing importantly to greater dependability of operation and ease of control.

The temperature and time are selected to give the desired amount of coating. While the limits of these variables are not extremely critical, applicants experience has shown that to obtain regularly and with certainty the best results in commercial practice, the temperature should be not less than 1400 F. nor above 2200 F., and that the treating time may vary according to thickness of coating desired. Except where a very thin coating is acceptable, less than onequarter hour treatment does not give sufficient siliconization and beyond 60 hours the outer portion of the coating may not be satisfactory.

In order to meet the highest demands of the trade, the resistance of a siliconized article should be such that, when it is subjected to the action of boiling per cent aqueous sulphuric acid under the usual standard test conditions, the rate of weight loss does not exceed 0.00585 gram per square centimeter per day. Or, as it is perhaps more usually stated in commercial practice, the rate of penetration of the case should not exceed about 0.12 inch per year in the standard boiling 30 per cent H2804 test. siliconized ferrous articles produced in accordance with the present invention not only consistently meet this standard of corrosion resistance but commonly far surpass it.

It has already been pointed out hereinabove that the treating atmosphere should be nonoxidizing in character. While it has been suggested heretofore that the presence of some hydrogen may be desirable because of its reducing effect, there seems to have been no previous recognition of the importance of excluding oxygen, whether free or combined, from the treating atmosphere to any such extent as the present applicants have found is essential to the attainment of satisfactory results in siliconizing metal articles. Even commercially pure hydrogen ordinarily contains more oxygen, free or combined, than is permissible for the purposes of the present invention. Applicants have found that the oxygen content of the hydrogen or hydrogen-containing gaseous medium at the time it is mixed with the silicon chloride vapors in the treating mixture supplied to the reaction zone must not be substantially in excess of that represented by or equivalent to a dew-point of 20 F. as a maximum, assuming all the available oxygen content to be in the form of H20, and still better not higher than 30 to 40 F. Said medium, when mixed with the silicon chloride vapors must be virtually free of carbon dioxide (CO2), and should not contain carbon monoxide (CO) in excess of about 2 per cent by volume, both these compounds, especially the carbon dioxide, being oxidants under the operating conditions characterizing the novel process. Moisture (H2O) is tolerable only to an extent represented by the above designated maximum dew-point of 20 F.

Since hydrogen or hydrogen-containing gases commercially available ordinarily contain oxygen, whether free or combined, in greater proportion than the permissible maximum hereinabove specified, and since the use of hydrogen from commercial sources is practically mandatory from a cost standpoint, a purifying treatment of the commercially available hydrogen or hydrogen-containing gaseous medium that is to provide the hydrogen component of the treating atmosphere, is usually an important feature of thenew process in actual practice. Ensuring the absence of oxygen in proportion greater than the very small proportion hereinabove specified as a permissible maximum, in conjunction with the maintenance of theproportioning of hydrogen to silicon chloride within the range limits herein disclosed, together. constitute an essential combination of factors to which are evidently attributable'in large measure the superior results obtainable in practicing the present invention.

It' has also been found by'applicants that the presence in the hydrogen-containing gaseous medium of a limited amountof a hydrocarbon gas, such as methane, ethane 'or the like, up to about 10 per cent by volume; is generally not harmful; nor is a much larger proportion of nitrogen, as has already-been pointed out. To the extent indicated, both nitrogen .and such hydrocarbon gas appear to constitute merely inert diluents of the hydrogen in'thecomplet treating atmospherein the reaction'zone. Therefore, in place of commercially pure hydrogen, which is comparatively expensive despite the fact that it generally still containsresidualoxygen and water vapor to an extent necessitating further purification before it is'usable in the present process, various and much cheaper industrial gas mixtures containing hydrogen can, under proper conditions, be successfully employed in practicing the 'invention, providedthe hydrogen constitutes at least 10 per cent ofthe mixture by volume, with an. inert diluent gas constituting practically all the remainder. Examplesof such available gas mixtures are cracked ammonia,

charcoal gas, and certain commercial generator gases high in nitrogen. It has been found that gases of thesetypes, as'ordinarily produced industrially, must in many cases bev purified in order to. render them suitable hydrogen-containing media for employment in the present process. As a rule, they contain, in addition to moisture and other forms of oxygen, varying proportions of carbon dioxide and monoxide, both also readily removable by known purification methods.

In practicing the invention, various forms of apparatus systems or plantsdiffering widely in specific details may be'utilized. In general, and assuming that the hydrogen or hydrogen-containingmedium' to be employed 'must be purified in order to meet the requirementshereinabove set forth, the apparatus should ordinarily include the following: Means for supplying and metering the hydrogen or hydrogen-containing gaseous medium; means for purifying the same to eliminate or sufficiently reduce its content of obiectionable components; means-for testing and indicating-the degree of purificationfrom atleastone of such components, as a control measure;

' source-of silicon' chloride vapors or gasand means for metering, directly or indirectly, the quantity thereof to be mixed with. the purified hydrogen-containing medium; provision for effecting suchmixture of the hydrogen-containing gas and silicon chloride vapors; amuflle furnace arranged to receivev suchgaseous mixture and providing a high-temperature reaction zone in which the artic1es=to be siliconizedare treated under controlled conditions; and-means for trapping out and recovering any excess silicon chloride contained in the gases exiting, from the furnace, and for leading-away and discharging from the system any. finally remaining gases; the apparatus system as a' whole being" protected is against accidental ingress of air into any part thereof.

In further explaining the principles of the invention, reference will be made to the accomi panying drawings in which Fig. 1 illustratesdiagrammatically one type of plant or apparatus system which can be employed in practicing the invention; and

Figs. 2 to 6 are photomicrographs showing the microstructure of the siliconized case and its bonding to the underlying core obtained in typical instances when siliconizing ferrous metal articles by the novel process.

In the illustrative example how to be described,

-it will be assumed that the starting materials employed in forming-the treating atmosphere are commercial silicon tetrachloride and-commercial hydrogen. This commercially pure hydrogen,

ordinarily supplied under: pressure in theusual steel cylinders or bottles, contains some oxygen and water vapor, as hereinbefore pointed out. Before it is mixed with silicon tetrachloride vapor or gas,the hydrogenis therefore passed through a high temperature combustionzoneto eifect combination of its free oxygen content With hydrogen; after which the resultant. H2O, together with any other moisture originally present in the.

commercially pure gas, is removed-by appropriate drying means tosuch an extent that the dewpoint of the purified gas does not exceed -20 F.

Referring to Fig. 1, commercially pure hydrogen passes from cylinder 2. into supply line 3 which is provided with-a suitable needle valve control indicated at 4-, Beyond the needle valve,

thesupply line is provided with a pressure relief device-5, here shown as aliquid seal; Beyond. this is arnetering device which, in this instance,,

is aflow meter of well known type, comprising a standard orifice 6-, through which the hydrogen flows, together with the U.-tube manometer or differential pressure indicating gauge 5a; the limbs of the U-tube being. in communication with opposite sides of the standard orifice. The metered hydrogen passes next to the combustion I furnace, here shown as arefractory muffle tube 1- heateolbymeans of a resistance winding 8 and housed in a heat-insulatingjacket 9. By appropriate. control means of conventional character, flow of currentthrough the Winding 8 may be accurately and automatically controlled to maintain the temperature within the combustion furnace at the desired point, typically at around 1650 F. In this combustion chamber, which is desirably packedwith ferro-silicon, any free or elemental oxygen contained in the commercial hydrogenis burned to water.

From the combustion chamber, the hydrogen undergoing purification passes through pipe i0 into and through a. drying train here shown as consisting of columns or towers ii, i2, connected in series by pipe i 3, these.conta ning one or more suitable drying agents for removing from the.

system here illustrated such provision is made at another location, as will presently appear. Any suitable means may be employed for continuously supplying the proper proportion of silicon tetrachloride vapor to be mixed with the metered and purified hydrogen in providing the complete treating atmosphere wherein the weight percentage relation of hydrogen to silicon tetrachloride shall have any predetermined numerical value within the limits of the percentage range herein disclosed as essentially characterizing the invention. If desired, the silicon tetrachloride may be first vaporized and the resultant vapor itself metered. Or, as in the illustrative example here given, such metering of the desired proportion of silicon tetrachloride vapors may be accomp ished indirectly by metering the silicon tetrachloride in liquid condition. In the arrangem nt i lustrated, liquid silicon tetrachloride is contained in a closed receptacle l1, and the purified hydrogen is discharged throu h dip p pe l8 terminating near the bottom of the receptacle in o the body of liquid silicon tetrachloride and bubbles upwardly therethrough, thus becoming saturated with silicon tetrachloride vapor at the prevail ng temperature, which may be exactly controlled at the desired point above outside room temperature by means of a heating resistance winding I9, the flow of current through which may be regulated and automatically controlled by t e usual rheostat and thermostat devices, not shown. The temperature of the liquid si icon tetrachloride may thus be mainta ned constant at any desir d point between room t mperature and the boiling point of silicon tetrachloride, whereby the desired proportioning of hydrogen and silicon tetrachloride vapors in the treating atmosphere supplied to the reaction zone may be exactly re ulated and controlled.

The purified hydrogen gas charg d with SiCh vapor to the desired predetermined extent exits from the space above the liquid level in the bubbler or vaporizer I! through pipe 20, the mixture passin thence through line 2 i into the treating or siliconizing chamber 22. With a fixed liquid evaporative surface in the container I1, it becomes possible to adiust the temperature of the liquid silicon tetrachloride and the rate of flow of hydrogen through the container in such manner as to obtain the d sired wei ht p rcenta e relation of hydrogen to silicon tetrachloride in t e gaseous mixture passing to the siliconizing chamber. The desired pr determined percenta e relation having been e tablished, said temperature and rate of flow may each then be maintained subs antially constant during the siliconizing operation.

The siliconizing or treating chamber 22 is a furnace muiile of sufficient size to accommodate the article or art cles to be siliconized, with some space to spare, and may be of any suitable type and construction. It is adequately heat-insulat d, as indicated at 23, and in this instance is electrically heated by external resistance winding 24; suitable temperature adjusting and control means (not shown) being provided, as in the case of the furnace I, including thermostatic control means. The walls of the chamber should be of refractory material such as si ica, and as here shown the chamber is a silica tube substantia lv circular in cross-section. Residual gases exiting from the reaction zone leave the siliconizing chamber through exit pipe 25 which is arranged to dischar e the efiiuent spent gas mixture through the dip pipe 25 at a point well below the level of a suitable liquid reagent, such as concentrated sulphuric acid, contained in a receptacle 21. This serves both as a liquid seal and also as a means of trapping out most of whatever silicon tetrachloride may be contained in the eflluent gas mixture, while permitting any unused hydrogen to escape from the space above the liquid seal, along with reaction by-products, through exit pipe 28. This arrangement also provides for maintaining a slight back pressure on the system.

For determining the dew-point of the purified hydrogen, the following provision is made in the particular form of apparatus system here illustrated: By means of pipe 29 and three-way valves 30 and 3|. pipes 16 and 2! may be directly connected. W th the valves 30 and 3| properly set to afford such direct connection, metered and purified hydrogen may thus be Icy-passed around the silicon tetrachloride vaporizer and sent directly to and through the siliconizing chamber 22. A branch pi e 32 leading from exit tube 25, is provided with a nozzle end 33, adapted to discharge a jet of gas against the mirror 34 of a dew-po nt meter of conventional type indicated generally at 35 and including means (not shown) for cooling the mirror down to the temperature at which dew will be deposited thereon by such gas, and indicating that temperature. Shut-off valves 36 and 3'! are provided in pipes 26 and 32, respectively.

In carrying out the process with the aid of the apparatus system illustrated, the article to be siliconi ed is first properly placed and supported within the treating chamber 22, in such manner that it is spaced from the furnace walls, most desirably as symmetrically as is feasible. In order not to interfere with the siliconizing action on the article where it is contacted by the supporting means, the latter should be so fashioned as to provide virtual point support only. The supportin means may advantageously be machined from black carbon or graphite which. besides resisting attack by the treating atmosphere. is porous enough to permit access by the treating atmosp ere to the article surface at such contact point.

The next step is to purge the treating furnace chamber of air and make certain that the purifying and drying train is workin roperly, so that the ab lity to supply substantially oxygen-free hydrogen (or other hydrogen-containing gas) as a component of the com lete treatin atmos here may be ensured. W ile any suitable inert gas may be used for this purging operation. the hydrogen-containing gas itself. assumed to be commercial hydrogen in this specific example, is convenient to use for this pur ose, the test for proper functioning of the purifying and drying train being conducted simultaneousl therewith. To this end, the combustion furnace 1 is brought up to the proper reaction temperature for effecting combustion of free oxygen into water vapor, and valves 39 and 3! are properly set to by-pass the purified hydrogen around the silicon tetrachloride metering and mixing unit [1, directly to the sil conizing chamber 22. At the same time, cut-off valve 36 is closed or sufliciently throttled down, while cut-ofi valve 31 is opened. The pressure-control and regulating needle valve 4 of the gas cylinder 2 of compressed hydrogen is now opened to permit flow of hydrogen at the desired rate. The hydrogen flows through the metering unit, combustion chamber, drying train, and reaction or siliconizing chamber, successively, exiting satisfactorily as evidenced by constant indication of the desired low dew-point of the effluenthydrogen gas, the temperature within the treating chamber 22, heating of which may have already been started, is brought up to the required operating point, between 1700' and 1900 F. being usually most desirable. Valve 3'! in the dew-point testing branch line 32 now being closed, and valve 30 being opened as far as may be necessary to permit the desired degree of free gas flow through the system, valves 30 and 3! are now set to close off by-pass line 20 and compel the deoxygenated and dried hydrogen leaving tower or column 3 2 to pass through the silicon tetrachloride bubbler or vaporizer unit as already described. Normally, the temperature of the liquid silicon tetrachloride in the bubbler container is a predominant factor in determining the desired weight percentage relation of hydrogen to silicon tetrachloride in the treating atmosphere supplied to the-treating chamber 22, and the pressure at pressure-control valve l determines the rate of flow of the treating atmosphere through the treating chamber and over the surface to be siliconized. By a proper adiustrnent of said pressure-control valve and of the heating of the container ii, the aforesaid percentage relation of hydrogen to silicon-tetrachloride, as well as the desired rate of flow of the treating atmosphere over the surface to be siliconized, can be obtained.

After the treatment has contin-ued'for a. sufficient length of time to produce the desired thickness of siliconized coating or case, the flow of hydrogen is shut oif by closing control valved in the supply line, and heat is cut ofi' from the treating chamber, the article being permitted to cool to some extent at least before opening the siliconizing chamber.

In practice. it has been found that where the siliconizing operation has been properly conducted in accordance with the principles of the invention herein disclosed; the silico-nized article may be withdrawn from the treating chamber to the air while still hot, with no danger of injury to the case, especially where the case is not extremely thick, say, not over 0.10 inch, for example, and especially where the temperature in the reaction or siliconizing zone has been maintained not substantially higher than around 1750 to 1800 F. Employment of'such'relatively moderate siliconizing temperatures has been found to favor production of a case that is particularly satisfactory in respect both to corrosion-resistance and to resistance to thermal shock as well.

Where the case is relatively very thick, the article may desirably be cooled to about normal atmospheric temperature in a neutral or'reduc ing atmosphere, either in the siliconizing chamher or in a special cooling chamber into which the siliconized article may be transferred with out exposing it to an oxidizing atmosphere. In

. other instances, the article may be quickly with.-

drawn. when only partly. cooled, andthen'suit 14* ably cooled'further by quenching in a suitable bath such as oil.

In actual practice, it has been found that when subjecting the work to the described siliconizing treatment for about three hours, the best resistance to corrosion is obtained by so maintaining the proportioning of the components of the treating atmosphere entering the reaction zone that the weight of the free hydrogen present is between about 0.06 and about 2.0 per cent of the weight of silicon tetrachloride. just mentioned, hydrogen percentages (based on silicon tetrachloride) of from 0.1 to 1.4 are ordinarily most consistently effective in enabling production of cased articles that are satisfactory from the standpoint of all around performance. However, acceptable results are obtainable with the novel process when employing hydrogen percentages on the stated basis that are considerably higher than 2.0 and even above 2.8. But we have found that, at percentages higher than 4.0, the resultant'siliconized articles lack the char acteristics essential to practical utility.

The elTect of varying the percentage weight relation ofhydrogen to silicon tetrachloride in the treating atmosphere is illustrated by the data given in Table I below. In each instance the article was subjected" to the siliconizing treatment for 3 hours at 1950 F.

Table I Acid resistance, wt. Wt. change during chan e after 10 days in Atmosphere, siliconizing (gJcmJ) boiling 30% 112304 Hs/SiCll (g/cmfi) by Wei ht, Per Cent 8. A. E Gray cast S. A. E Gray cast 1045 steel iron 1045 steel iron 0.2330 Poor 0 0412 -0. 2318 +0. 0021 0 0137 -0. 1943 -0. 0067 0. 0228 +0. 0594 +0. 0121 1 Poor +0. 0625 1 Poor 1 Poor 1 Case cracked.

As indicated by the weight changes resulting from thesiliconizing treatment, increasing the percentage of hydrogen in the treating atmosphere favors the deposition reaction, the negative weight changes that occur with the lower hydrogen percentages eventually becoming positive in the higher percentages. Generally speaking, this means a corresponding increase in the silicon content of the article cases or coat ngs produced. It is evident that the quality of the coating, as determined by the standard corrosion test in boiling 30% sulphuric acid hereinabove referred to, is much too greatly reduced if the hydrogen weight percentage with reference to silicon tetrachloride is increased to 6%; while at a-hydrogen percentage of 2.8, the quality is still very good in the case of the 1045 steel specimen butis not goodin the case of the cast iron specimen. Applicants experience with the process shows that consistentl inferior results, even in the ca'seof steel, are obtained when the hydrogen percentage ishighe'r than 4%; From this table itwill also be observed that when the hydrogen percentage, is as low as 0.02 it is still possible to obtain a-satisfactorilycorrosion-resistant case on g-ray'cast ironibut not on S; A. E. 1045 steel; so that for practical purposes this isproperly to be regarded as 'thelowest permissible hydrogen percentage within. thescope. of the invention. Although, in general,.increasingxthe hydrogen per-P Within the range centage increases the overall percentage of silicon in the case, this increase of silicon content cannot be carried too far without such a sacrifice in quality of the case as to render it commercially valueless. However, it is feasible in practicing the novel process to obtain a relatively high percentage of silicon in cases produced on ferrous metal articles by employing in the treating atmosphere a relatively high ratio of hydrogen to silicon chloride, within the limits herein specified, while maintaining the temperature in the reaction zone at between about 1700 F. and about 2000 F. Under such conditions, the deposition reaction evidently predominates over the replacement reaction, although not necessarily to the extent of causing an increase in weight of the coated article over its weight prior to being treated. Assuming careful observance of the requirement for substantial absence of oxidants from the treating atmosphere, as well as virtually complete dryness thereof, it is feasible to produce consistently cases or coatings that are relatively high in silicon and of excellent quality in respect both to corrosion resistance and resistance to thermal shock. Thus, commercially satisfactory siliconized ferrous articles have been produced by the present process having as high as 26.7 per cent silicon in the outer layer of the siliconized case. From this percentage and the change in weight of the article from its original weight, it can be calculated that in such instances about 38.7 per cent of the silicon in the case results from the replacement reaction and 61.3 per cent is attributable to the deposition action.

Commercially satisfactory ferrous metal -articles having cases that contain as much as 16 per cent silicon in the outer layer thereof, and having an overall silicon content (1. e. average for the entire case) of 14.5 silicon or better, are consistently producible by the present process when operating at siliconizing temperatures of from 1700 to 2000 F., with the percentage weight ratio of free hydrogen to silicon tetrachloride in the entering gas mixture maintained r at between 0.1% and 2.0%. In a typical instance, where the article is composed of Armco iron and is treated for three hours at 1950 F., with the aforesaid hydrogen weight percentage in the entering gas mixture approximating 0.30%, the resultant case is about 0.06 inch thick, and the silicon content thereof in four successive depth zones or strata each 0.015 inch thick, beginning with the outermost, is substantially as follows: 15.9%, 14.8%, H.472; and 14.0%, respectively. Such a product shows high corrosion resistance, as well as excellent resistance to thermal and mechanical shock.

The coatings or cases produced on ferrous metal articles by the novel process are quite diff rent in their physical as well as chemical characteristics from those obtained by prior processes which involve embedding the article in comminuted solid silicon-containing material. Comparative tests show that a ferrous metal art cle coated by such a prior process. if immersed in water and the water is then slowly heated to the boiling po nt, will exhibit appreciable rusting. Also, the coating is relatively soft and easily cut with a file. Corresponding articles siliconized by the present process, when similarly treated, do not show rusting; they are also file hard.

Applicants have found that, with ferrous metals, the carbon content of the metal has considerable effect upon the corrosion resistance of the siliconized article, the resistance of siliconized articles of iron or steel high in carbon being less than where the carbon content is low. However, the rate of corrosion of siliconized ferrous metal articles produced in accordance with the principles of the present invention, even when the carbon content is high is consistently lower than the commercially permissible loss of 0.00585 gram per square centimeter per day in the standard boiling 30 per cent H2804 test.

While corrosion resistance may be regarded as the major criterion of a satisfactory siliconized case, it is also essential from the standpoint of commercial utility that the case be adequately resistant to thermal and mechanical shock. Articles of S. A. E. 1045 steel and of gray cast iron, for example, siliconized by the present process and subjected to as many as 750 cycles of alternate heating and quenching, each. cycle consisting of heating at 1000 F. for 18.5 minutes followed by quenching in water at 70 F. for 1.5 minutes, still show good corrosion-resistance after immersion for .24 hours in boiling 30% H2804. This shows there is no impairment of either the case itself or its bonding to the underlying core by the severe strains imposed upon both in such a test. It is also indicative of high resistance to mechanical shock because of the actually greater severity characterizing the thermal shock in the described test, as compared to any mechanical shock likely to be encountered in practical use.

Because of the excellent resistance to thermal shock characterizing articles produced in accordance with the invention, it is possible in practicing the present process, as already pointed out hereinabove, to withdraw a properly siliconized article from the treating furnace chamber and expose it to the outside air while still Very hot, with no danger of injury to the protective coating of the article, where the silicon-containing case is not extremely thick, not over about 0.10 inch, for example. Where the case is relativel very thick, it is usually better practice to cool the article in a neutral or reducing atmosphere to about normal atmospheric temperature, either in the siliconizing chamber or elsewhere. In other cases, the article may be quickly withdrawn when only partly cool, and suitably quenched in a bath of material inert thereto, such as oil.

Typical results in siliconizing articles of thin cross-section composed of steels containing substantial proportions of allo ing elements, as well as the siliconization of wholly non-ferrous materials, are illustrated by the data set forth in subjoined Table II. In each instance, the siliconizing treatment was at approximately 1750 F. and continued for 5 hours, the weight of free hydrogen in the entering gas mixture being about 0.23 of the weight of the silicon tetrachloride.

As indicated by the above data, siliconization markedly improves the resistance of 18-8 stainless steel to corrosion in boiling 20% hydrochloric acid. Highly resistant coatings are also produced on non-ferrous metals, such as Monel and pure nickel. However, siliconized cases produced on more massive articles composed of either ferrous or non-ferrous metal high in nickel are apt to be more brittle and less well bonded to the base metal than where the base metal is not high in nickel.

It is to be understood, of course, that the invention contemplates not only the production of a siliconized case united. to an unsiliconized core or base, but also siliconization of sheet metal, for example, throughout its thickness, for production of corrosion resistant articles of thin cross-section. Such completely siliconized products have important utility for certain purposes, and they can be produced with marked successby the present process.

Figs. 2-6, inclusive, of the drawings are representative photomicrographs showing the microstructure of the silicon-containing case produced by the novel process in typical instances, and the nature of its bond or union with the underlying core, all the views being sections taken through the case-core region, and the section having received a Nital etch after polishing, in each instance. The siliconized articles corresponding to these drawings, and the materials of which they are composed, are further identified in the following table.

18- v to be mistaken for particles of carbon or graphite. The dark areas 43 in the core are grains of pearlite.

The above-mentioned migration of carbon during the siliconizing treatment is still more pronounced when treating high carbon steels, but by carefully adjusting and controlling the composition of the treating atmosphere and the other conditions of operation in accordance with the invention, the tendency for such great concentration of carbon at the interface as to result in a defective bond and unsatisfactory resistance to corrosion and to thermal shock can be prevented.

Referring next to Figs. 3 and 4, which represent sections, at different magnifications, of siliconized malleable iron specimens having the same analysis, the interface between case and core is indicated at 44 and 45, respectively. It is evident that, in each instance, the inner portion of the case merges irregularly with the core, giving an integral bond or union between the case and malleable-iron core of highly satisfactory character. The temper carbon in the case and in the core is easily visible as indicated at 46 and 61, respectively. The higher magnification of Fig. 4 also enables the pearlitic structure of the core to be seen, as at 48. It will be noted that, in contrast to the siliconized steel article represented by Table III Composition Magnifig g a cation Shape Type of Material ((118.1115) (3 Si Mn S P 2 200 Rod"... Cold rolled low carbon steeL Not over 0.10.. (Usual low oercenzages) 3 }Bar Malleable iron 2.40 1.10 0.75 0.10 0.11. 100 do Gray cast iron 3.14.-. 2.75 1.0 0.03

Referring first to Figs. 2, 3 and 4, the articles represented thereby were siliconized in accordance with the procedure herein disclosed by maintaining them in the siliconizing muffle at a temperature of 1750" F. for at least three hours, the treating atmosphere supplied to the reaction zone consisting of commercial silicon tetrachloride and purified commercial hydrogen, the weight of hydrogen being 0.93% of the weight of silicon tetrachloride.

In Fig. 2, it will be noted that immediately subadjacent the interface all between the case and low carbon core, there is a limited region H showing a relatively high concentration of carbon, indicating formation of a thin encasing layer of relatively high carbon steel about the main part of the low carbon body or core. This is due to inward migration of carbon from the surface layers of the steel article as the siliconizing operation proceeds. The manner and extent to which this migration and concentration of carbon at the interface occurred in prior proposed processes is probably one reason why those processes failed to produce siliconized articles satisfactorily resistant to corrosion and also to thermal and mechanical shock. The bond between case and Fig. 2, there is substantially no concentration of carbon at the case-core interface.

Referring now to Figs. 5 and 6, which representa typical product obtained'by siliconizing gray cast iron by the novel process, these two figures, taken together, represent one section through the entire siliconized case and the casecore region, the line L--L passing through identical points in both figures. In producing this case in accordance with the novel process, com mercial silicon tetrachloride and purified commercial hydrogen were employed, the weight of hydrogen being 0.21 of the weight of silicon tetrachloride in the mixture entering the reaction zone which was maintained at substantially 1760 F. during the treatment of the cast iron article which was continued for four hours. It is evident that the bond or interface 49 between the siliconized case and core is integral and shows no cleavage, intimate interlocking of case and core constituents to provide an integral bond being evident. This is a particularly surprising result in view of the admitted impossibility of producing satisfactorily siliconized ordinary gray cast iron by methods heretofore proposed. Par-- ticles of g aphite in case and core are indicated at 50 and 5!, respectively. As in the specimen represented by Figs. 3 and 4, substantially no concentration of carbon appears at the case-core interface.

It is also apparent from Fig. 5 that the siliconized case consists in this instance of a relatively thin outer portion 52 and a thicker inner portion 53 which, although merging or blending one into the other without a distinct break, in such a way as to render the case as a whole integral, are nevertheless distinguishable as layers having a discernible interface at 54. The dark areas 42a in the outer layer 52 of the case are polishing pits or voids, similar to pits 42 in Fig. 2. The outer layer 52 is higher in silicon than the inner layer, usually containing over 15% silicon; while the inner layer 53 commonly has an average or overall silicon content of around 14.0 to 14.5%. The crystal structure of the two layers is different, as would be expected from a consideration of the iron-silicon constitution diagram (see, e. g., Metals Handbook, 1939 edition). This integrally united two-layer formation is generally characteristic of high-silicon cases produced on steel, as Well as cast iron, in practicing the present invention, and is of distinct advantage. Integral union of the two layers, with consequent freedom from tendency of the outer layer to spall off, is achieved with greatest certainty when operating under the hereinabove indicated optimum conditions of siliconizing temperature and proportioning of free hydrogen to silicon tetchrachloride, especially if a relatively low siliconizing temperature be employed, without forcing the siliconizing too rapidly, and if the weight percentage of hydrogen to the silicon chloride does not exceed 2.0%. Slow cooling of the siliconized article at the conclusion of the siliconizing operation is also helpful. Unless the conditions of operation herein disclosed as characterizing the present novel process are adhered to, cleavage between the layers of a two-layer iron-silicon case, where such a case is formed at all, is virtually sure to occur, with disastrous results.

The feasibility, already mentioned, of replacing commercially pure hydrogen, either wholly or in part, by industrial gas mixtures consisting mainly of free hydrogen and nitrogen, while maintaining the Hz/SiCh ratio within the hereindisclosed limits which applicants have found to be critical, is of important advantage in practicing the novel process, not only because of the saving in cost that can be thus effected but, in addition, because the presence of the inert diluent gas in the siliconizing atmosphere appears in many instances, to affect favorably the progress of the siliconizing reactions. This may be due in part to the fact that the lower concentration of the essential atmosphere components, free hydrogen and silicon chloride, makes for somewhat lower velocity and greater smoothness of the reactions, while also enabling increased flexibility of operating control in respect, for example, to the rate of atmosphere flow over the surface of the work being treated in the siliconizing furnace chamber. Cracked ammonia is an especially desirable substitute for commercially pure hydrogen for the purposes of this invention. Although by special treatment in known manner it can be had in forms containing much less than 75% H2 that may be employed in the present process, cracked ammonia as initially produced is a gas mixture containing approximately 75% hydrogen by volume, the balance being chiefly nitrogen with sometimes a small percentage of carbon monoxide and carbon dioxide. These latter impurities are readily removed by usual methods, thereby giving a purified gas mixture of about 74.6% hydrogen, and 25.4% nitrogen as inert diluent. Employing this purified hydrogen-containing medium in mixture with silicon tetrachloride vapor in such proportions that the free hydrogen content of the treating gas mixture entering the muiile or reaction chamber is, for example, from about 0.35% to about 0.45%, by weight, of the silicon tetrachloride, and employing a siliconizing temperature in the mufiie chamber of approximately l700 to 1750 F., with a treating time of 5 to 6 hours, excellent results have been obtained in siliconizing ferrous metal articles. Among the articles that have been very successfully processed in this manner are various pump parts, including pump sleeves or shrouds made of hot rolled low carbon steel, and split gland flanges made of cast steel. The siliconized articles thus produced possess excellent resistance to corrosion as Well as to thermal shock. Not only is the siliconized case firmly bonded to the core metal, but the outer layer of the case is firmly united to the inner layer and exhibits no tendency to spall off. In a typical instance, articles siliconized with the aid of cracked ammonia as above described, when tested by boiling in 30% H2804 for 21 days, showed no signs of case or coating failure, and the rate of corrosion was below the commercial tolerance limit.

It is to be understood that the above specified ranges of 0.35% to 0.45% for the Hz/SiCh ratio, and of 1700 to 1750" F. for the siliconizing temperature are only intended to be illustrative of what has been found to be good practice when cracked ammonia of the stated composition is employed. Satisfactory results are obtainable under other conditions, when employing cracked ammonia or other H2N2 mixtures of varying specific composition, within the operating limits hereinabove set forth in describing the use of commercially pure hydrogen.

What is claimed is:

l. The process of providing a suitably heatresistant article with a silicon-containing coating or case that is resistant to corrosion and thermal shock, which comprises maintaining said article in a reaction zone at a temperature of at least about 1400 F. but below a temperature sufficiently high to injure the article, while subjecting it to the action of a treating atmosphere substantially free from oxygen, both elemental and combined, and containing both silicon chloride and free hydrogen as essential components, the proportioning of said components in. the atmosphere as supplied to the reaction zone being such that, expressing the silicon chloride component in terms of silicon tetrachloride, the weight of the free hydrogen component is between 0.02 and 4.0 per cent of the weight of the silicon chloride component.

2. The process set forth in claim 1, which further includes flowing the treating atmosphere over the surface of said article rapidly enough to prevent undesirable concentration of byproducts in the reaction zone.

3. The process of siliconizing an article composed of a metal reactive with silicon chloride or capable of alloying with silicon, which comprises maintaining said article in a reaction zone at a temperature of at least about 1400 F. but below the fusion point of the article, while flowing over the surface of said article a treating atmosphere substantially free from oxygen, both elemental and combined, and containing both silicon chloride and free hydrogen as essential components, the proportioning of said components in the atmosphere as supplied to the reaction zone being such that, expressing the silicon chloride component in terms of silicon tetrachloride, the weight of the free hydrogen component is between 0.02 and 4.0 per cent of the 76 weight of the silicon chloride component.

4. The process of siliconizing a ferrous metal article to provide the same with an adherent protective case that is resistant to corrosion and thermal shock, which comprises maintaining said article in a reaction zone at a temperature of at least about 17 F. and not substantially higher than 2000 F., while flowing over the surface of said article a treating atmosphere substantially free from oxygen, both elemental and combined, and containing both siliCOn chloride and free hydrogen as essential components, the proportioning of said components in the atmosphere as supplied to the reaction zone being such that, expressing the silicon chloride component in terms of silicon tetrachloride, the weight of the free hydrogen component is between 0.02 and 4.0 per cent of the weight of the silicon chloride component.

5. The process of siliconizing a ferrous metal article to provide the same with an adherent protective case that is resistant to corrosion and thermal shock, which comprises maintaining said article in a reaction zone at a temperature of at least about 1700" F. and not substantially higher than 2000 F. while flowing over the surface of said article a treating atmosphere substantially free from oxygen, both elemental and combined, and containing both silicon chloride and free hydrogen as essential components, the proportioning of said components in the atmosphere as supplied to the reaction zone being such that, expressing the silicon chloride component in terms of silicon tetrachloride, the weight of the free hydrogen component is between about 0.06 and about 2.0 per cent of the weight of the g silicon chloride component.

'6. The process of siliconizing a ferrous metal article to provide the same with an adherent protective case that is satisfactorily resistant to corrosion and thermal shock, which comprises maintaining said article in a reaction zone at a temperature of at least about 1700 F. and not substantially higher than 2000 F. while flowing over the surface of said article a treating atmosphere substantially free from oxygen, both elemental and combined, and containing both silicon chloride and free hydrogen as essential components, the proportioning of said components in the atmosphere as supplied to the reaction zone being such that, expressing the silicon chloride component in terms of silicon tetrachloride, the weight of the free hydrogen component is between 0.1 and 1.4 per cent of the weight of the silicon chloride component.

7. The process set forth in claim 5, wherein said treating atmosphere consists of a mixture, in the proportions stated, of silicon chloride and commercially pure hydrogen that has been further purified to render it substantially free from residual free oxygen and moisture.

8. The process set forth in claim 6, wherein said treating atmosphere consists of a mixture, in the proportions stated, of silicon chloride and commercially pure hydrogen that has been further purified to render it substantially free from residual free oxygen and moisture.

9. The process set forth in claim 1, wherein said treating atmosphere also contains an inert diluent gas in proportion not substantially exceeding nine times that of said free hydrogen by volume.

10. The process set forth in claim 1, wherein said treating atmosphere also contains an inert diluent gas in proportion not substantially exceeding that of said free hydrogen by volume.

11. The process set forth in claim 1, wherein said treating atmosphere also contains an inert diluent gas in proportion not substantially exceeding nine times that of said free hydrogen by volume, said diluent gas consisting chiefly of nitrogen.

12. The process set forth in claim 5, wherein said treating atmosphere also contains an inert diluent gas in proportion not substantially exceeding that of said free hydrogen by volume, said diluent gas consisting chiefly of nitrogen.

13. The process of siliconizing an article composed of a metal reactive with silicon chloride or capable of alloying with silicon, which comprises maintaining said article in a reaction zone at a temperature of at least about 14=00 F. but below the fusion point of the article, while flowing over the surface of the article a treating atmosphere consisting of silicon chloride and cracked ammonia that is substantially free from oxidants, the proportioning of silicon chloride and cracked ammonia in the atmosphere entering said reaction zone being such that it contains free hydrogen in amount equal to from 0.02 to 4.0 per cent, by weight, of the amount of silicon chloride calculated as silicon tetrachloride.

14. The process as set forth in claim 13, wherein the temperature in said reaction zone is maintained between approximately 1700 and 2000 F., and the free hydrogen content of the atmosphere entering the reaction zone is between 0.06 and 2.0 per cent, by weight, of the silicon chloride content calculated as silicon tetrachloride.

15. The process set forth in claim 13, wherein the temperature in said reaction zone is maintained between approximately 1700 and 1750 F., and the free hydrogen content of the atmosphere entering the reaction zone is between 0.35 and 0.45 per cent, by weight, of the silicon chloride content calculated as silicon tetrachloride.

ASHLAND S. HENDERSON. BRUCE W. GONSER. EDWARD E. SLOWTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,672,444 Beckett June 5, 1928 1,853,369 Marshall Apr. 12, 1932 2,114,802 Kinzel Apr. 19, 1938 2,236,209 Daeves Mar. 25, 1941 

1. THE PROCESS OF PROVIDING A SUITABLY HEATRESISTANT ARTICLE WITH A SILICON-CONTAINING COATING OR CASE THAT IS RESISTANT TO CORROSION AND THERMAL SHOCK, WHICH COMPRISES MAINTAINING SAID ARTICLE IN A REACTION ZONE AT A TEMPERATURE OF A LEAST ABOUT 1400*F. BUT BELOW A TEMPERATURE SUFFICIENTLY HIGH TO INJURE THE ARTICLE, WHILE SUBJECTING IT TO THE ACTION OF A TREATING ATMOSPHERE SUBSTANTIALLY FREE FROM OXYGEN, BOTH ELEMENTAL AND COMBINED, AND CONTAINING BOTH SILICON CHLORIDE AND FREE HYDROGEN AS ESSENTIAL COMPONENTS, THE PROPORTIONING OF SAID COMPONENTS IN THE ATMOSPHERE AS SUPPLIED TO THE REACTION ZONE BEING SUCH THAT, EXPRESSING THE SILICON CHLORIDE COMPONENT IN TERMS OF SILICON TETRACHLORIDE, THE WEIGHT OF THE FREE HYDROGEN COMPONENT IS BETWEEN 0.02 AND 4.0 PER CENT OF THE WEIGHT OF THE SILICON CHLORIDE COMPONENT. 